Without limiting the scope of the invention, its background is described in connection with the fabrication of high density dynamic random access memory (DRAM) devices.
Heretofore, in this field, thin film capacitors in DRAMs have used SiO.sub.2 or Si.sub.3 N.sub.4 as the dielectric material. As the density of integrated circuits (number of devices per square centimeter) increases, the capacitors which store electrical charge in each DRAM memory device are generally required to decrease in size while maintaining approximately the same capacitance. Referring to the following equation, C is the capacitance of a planar capacitor, .epsilon. is the dielectric constant, .epsilon..sub.0 is the dielectric permittivity of free space (a constant), A is the area, and d is the thickness of the dielectric. ##EQU1##
It is seen that the capacitance is directly proportional to the dielectric constant and inversely proportional to the dielectric thickness. Therefore, to build smaller capacitors while maintaining the same capacitance, one must increase .epsilon. and/or decrease the dielectric thickness d.
One method of allowing a decrease in the area of capacitors in DRAM cells is to make use of materials with a much higher dielectric constant than SiO.sub.2 or Si.sub.3 N.sub.4. The dielectric constant for both of these materials is less than ten. An important class of high dielectric constant materials is the perovskites (e.g. BaTiO.sub.3, SrTiO.sub.3, and (Ba,Sr)TiO.sub.3). The dielectric constants of these materials may be as high as 10,000 when they are fabricated as bulk ceramics. Other classes of materials with high dielectric constants are also being considered for use as the dielectric material in DRAM capacitors. To be useful in the manufacture of VLSI capacitors, these materials should be tolerant to the many VLSI processing steps which are used in the fabrication of DRAM cells.